This invention relates to digital protective relaying systems wherein various electrical values of an electric power system are converted into digital coded signals, and are used to operate protective relays for protecting the power system.
Heretofore known protective relaying devices utilizing analogue values obtained from current transformers (CT), potential transformers (PT) or a potential device (PD) and the like for the protection of electric power transmission lines, are roughly classified into distance relay devices, wherein electric values originated from only an electric station (or the own station) connected to one end of the power transmission line are used for the protection of the same, and phase comparison relay devices wherein the protection of the power line system is carried out by utilizing electric data of the own station and a remote station. In these conventional protective relay devices, the outputs of the CT, PT (or PD), and the like are converted, in the protective relay devices, into signals suitable for the protective devices used. For instance, in the phase comparison relay devices, the current data from the remote end is converted into a phase comparison signal, which is transmitted over a specially provided channel so that it is compared with a phase comparison signal originated from the own end power station for protecting the power line system.
In either of the relay devices, however, recent rapid increase in capacity of the power line system and the complication of the construction necessitated frequent change of the protective settings or of the type of the protective devices, such a change being found extremely troublesome.
On the other hand, along with the recent progress in digital computers, digital control of electric power systems has now become a common practice. In this control system, the analogue outputs from the CT, PT (or PD) are sampled, and converted into digital signals by an A/D converter, and the measurement, control, and the protection of the power system are accomplished by means of a digital computer and the like, utilizing digital processing techniques. In this manner, the measurement, control, and the protection can be attained by utilizing common data, thus reducing the required number of cables, and the aforementioned change of the settings and the like can be accomplished by modifying the software or the program.
Various digital protective relaying systems adapted for use in the computer controlled power line system have been developed in various countries. However, so far as we are aware, no fully established protective relaying system has not yet been proposed.